I used to think too that Euphorbias were cacti!
I thought just in a different Genus..well..now I know better
Come to think of it, I find Euphorbias favor a bit more watering than Cacti..so that is another thing I learned as I handled them this last couple of years.
But I still use my own simple determination..if it has spines particularly on the main stem..maybe a cacti..if the correct name is given then I can truly research it and confirm..if not, ask around or compare photos. If pointy things just at the leafy edge..most of the time succulents.